The percent hack is not part of the Internet mail standards

You've come to this page because you've made a claim similar to the
following:

MTA "xyz" does not conform to the Internet standards because it doesn't
support special meanings for the '%' (or '!') character by default.

This is the Frequently Given Answer to that claim.

Contrary to popular belief, the so-called percent hack is
not part of the Internet mail standards. Neither is
bang path addressing. '%' and '!' are just ordinary
characters with no special meaning as far as RFCs 821 and 822 are
concerned.

The only Internet standard that discusses special meanings for these
characters in mailbox names is RFC 1123, § 5.2.16. But that
section states (three times, no less) that it is specifically talking
about non-Internet mail environments.

In any case, UUCP-style mail addressing may have been
an important consideration for Internet back in 1989, when RFC 1123 was
published, but it is far less so now, given the widespread use of SMTP.

The
ORBS web page on
MTA security used to (before the disappearance of ORBS) make this
mistake of thinking that these characters are somehow special. It
criticised the qmail MTA for accepting messages that are
addressed to mailboxes whose local parts contain the '!' character. ORBS
called the '!' character a "standard network addressing indicator". But
it isn't.

qmail is perfectly within its rights to treat '!' exactly the same
as it treats any other character in a local part, and does not violate the
Internet mail standards in any way when doing so. It is ironic that an
MTA that actually implements the original vision of SMTP of a universal
directly connected mail transport system for Internet, without all of the
non-standard bodges and syntactic quirks such as the percent hack,
is actually criticised for doing so.